This invention is related to patent application Ser. No. 437,923, Johnson, now abandoned, and its c-i-p application Ser. No. 728,466, now abandoned, both to the assignee of the present invention, the disclosures of which are both hereby incorporated by reference.
The Johnson application discloses a counterrotating turbofan driven by a counterrotating power turbine. a feature of the Johnson application is the configuration of the power turbine and the ratio of the mean flowpath radius of such turbine to the mean flowpath radius of the gas generator. This is required in part to ensure lower fan tip speed and/or higher turbine blade speed.
Under certain design conditions, it may be desirable to reduce the mean flowpath radius of the power turbine. According to an embodiment shown in Johnson, such a reduction in the mean flowpath radius of the power turbine would increase the fan tip speed thereby reducing the efficiency of the engine.